Hominid or hominin?

Some scientists use a broader definition of Hominidae which includes the
great apes, and instead call the group I am discussing "hominins".
For a good discussion of the hominid/hominin terminology issue,
read this article
by Lee Berger.

The word "hominid" in this website refers to members of the family of humans, Hominidae, which
consists of all species on our side of the last common ancestor of humans and living
apes. Hominids are included in the superfamily of all apes, the Hominoidea,
the members of which are called hominoids. Although the hominid fossil record is
far from complete, and the evidence is often fragmentary, there is enough to give a
good outline of the evolutionary history of humans.

The time of the split between humans and living apes used to be thought to have
occurred 15 to 20 million years ago, or even up to 30 or 40 million years ago. Some
apes occurring within that time period, such as Ramapithecus, used to be considered
as hominids, and possible ancestors of humans. Later fossil finds indicated that
Ramapithecus was more closely related to the orang-utan, and new biochemical
evidence indicated that the last common ancestor of hominids and apes occurred
between 5 and 10 million years ago, and probably in the lower end of that range
(Lewin 1987). Ramapithecus therefore is no longer considered a hominid.

The field of science which studies the human fossil record is known as
paleoanthropology. It is the intersection of the disciplines of paleontology (the
study of ancient lifeforms) and anthropology (the study of humans).

The species here are listed roughly in order of appearance in the fossil
record (note that this ordering is not meant to represent an evolutionary
sequence), except that the robust australopithecines are kept together.
Each name consists of a genus name (e.g. Australopithecus,
Homo) which is always capitalized, and a specific name
(e.g. africanus, erectus) which is always in lower case.
Within the text, genus names are often omitted for brevity. Each species
has a type specimen which was used to define it.

This species was named in July 2002 from fossils discovered in Chad
in Central Africa (Brunet et al. 2002, Wood 2002). It is the
oldest known hominid or near-hominid species, dated at between
6 and 7 million years old. This species is known from a nearly complete cranium nicknamed Toumai, and a number
of fragmentary lower jaws and teeth. The
skull has a very small brain size of approximately 350 cc. It is not
known whether it was bipedal.
S. tchadensis has many primitive apelike features, such
as the small brainsize, along with others, such as the brow ridges and
small canine teeth, which are characteristic of later hominids. This mixture,
along with the fact that it comes from around the time when the hominids are
thought to have diverged from chimpanzees, suggests it is close to the
common ancestor of humans and chimpanzees.

This species was named in July 2001 from fossils discovered in western Kenya (Senut et al. 2001).
The fossils include fragmentary arm and thigh bones, lower jaws, and teeth and were
discovered in deposits that are about 6 million years old. The limb bones are about 1.5
times larger than those of Lucy, and suggest that it was about the size of a female
chimpanzee. Its finders have claimed
that Orrorin was a human ancestor adapted to both bipedality and tree climbing,
and that the australopithecines
are an extinct offshoot. Given the fragmentary nature of the remains, other scientists
have been skeptical of these claims so far (Aiello and Collard 2001). A later paper (Galik
et al. 2004) has found further evidence of bipedality in the fossil femur.

This species was named Australopithecus ramidus in September 1994 (White et al. 1994;
Wood 1994) from some fragmentary fossils dated at 4.4 million years.
A more complete skull and partial skeleton was discovered in late 1994
and based on that fossil, the species was reallocated to the genus
Ardipithecus (White et al. 2005). This fossil was extremely fragile,
and excavation, restoration and analysis of it took 15 years. It was published
in October 2009, and given the nickname 'Ardi'.
Ar. ramidus was about 120 cm (3'11") tall and weighed about 50 kg (110 lbs).
The skull and brain are small, about the size of a chimpanzee. It was bipedal on the
ground, though not as well adapted to bipedalism as the australopithecines were, and
quadrupedal in the trees. It lived in a woodland environment with patches of forest,
indicating that bipedalism did not originate in a savannah environment.

A number of fragmentary fossils
discovered between 1997 and 2001, and dating from 5.2 to 5.8 million years old,
were originally assigned to a new subspecies, Ardipithecus ramidus kadabba
(Haile-Selassie 2001), and later to a new species, Ardipithecus kadabba (Haile-Selassie et al. 2004).
One of these fossils is a toe bone belonging to a bipedal creature, but is a few
hundred thousand years younger than the rest of the fossils and so its
identification with kadabba is not as firm as the other fossils.

This species was named in August 1995 (Leakey et al. 1995). The material
consists of 9 fossils, mostly found in 1994, from Kanapoi in Kenya, and 12
fossils, mostly teeth found in 1988, from Allia Bay in Kenya (Leakey et
al. 1995). Anamensis existed between 4.2 and 3.9 million years ago,
and has a mixture of primitive features in the skull, and advanced features
in the body. The teeth and jaws are very similar to those of older fossil
apes. A partial tibia (the larger of the two lower leg bones) is strong
evidence of bipedality, and a lower humerus (the upper arm bone) is
extremely humanlike. Note that although the skull and skeletal bones are
thought to be from the same species, this is not confirmed.

A. afarensis existed between 3.9 and 3.0 million years ago. Afarensis
had an apelike face with a low forehead, a bony ridge over the eyes, a flat nose,
and no chin. They had protruding jaws with large back teeth. Cranial capacity
varied from about 375 to 550 cc. The skull is similar to that of a chimpanzee,
except for the more humanlike teeth. The canine teeth are much smaller than those
of modern apes, but larger and more pointed than those of humans, and shape of the
jaw is between the rectangular shape of apes and the parabolic shape of humans.
However their pelvis and leg bones far more closely resemble
those of modern man, and leave no doubt that they were bipedal (although adapted
to walking rather than running (Leakey 1994)). Their bones show that they were
physically very strong. Females were substantially smaller than males, a condition
known as sexual dimorphism. Height varied between about 107 cm (3'6") and 152 cm
(5'0"). The finger and toe bones are curved and proportionally longer than in
humans, but the hands are similar to humans in most other details (Johanson and Edey
1981). Most scientists consider this evidence that afarensis was still partially
adapted to climbing in trees, others consider it evolutionary baggage.

This species was named in 2001 from a
partial skull found in Kenya with
an unusual mixture of features (Leakey et al. 2001). It is aged about
3.5 million years old. The size of the skull is similar to A. afarensis and
A. africanus, and has a large, flat face and small teeth.

A. africanus existed between 3 and 2 million years ago. It is similar to
afarensis, and was also bipedal, but body size was
slightly greater. Brain size may also have been slightly larger, ranging between
420 and 500 cc. This is a little larger than chimp brains (despite a similar body
size), but still not advanced in the areas necessary for speech. The back teeth
were a little bigger than in afarensis. Although the teeth and jaws of
africanus are much larger than those of humans, they are
far more similar to human teeth than to those of apes (Johanson and Edey 1981).
The shape of the jaw is now fully parabolic, like that of humans, and the size of
the canine teeth is further reduced compared to afarensis.

This species was named in April 1999 (Asfaw et al. 1999). It is known from
a partial skull.
The skull differs from previous australopithecine species in the
combination of its features, notably the extremely large size of its
teeth, especially the rear ones, and a primitive skull morphology.
Some nearby skeletal remains may belong to the same species. They show
a humanlike ratio of the humerus and femur, but an apelike ratio of the
lower and upper arm. (Groves 1999; Culotta 1999)

A. sediba was discovered at the site of Malapa in South Africa in 2008. Two
partial skeletons were found, of a young boy and an adult female, dated between 1.78 and
1.95 million years ago (Berger et al. 2010, Balter 2010). It is claimed by its finders to be transitional
between A. africanus and Homo and, because it is more similar to Homo
than any other australopithecine, a possible candidate for the ancestor
of Homo. A. sediba was bipedal with long arms suitable for climbing,
but had a number of humanlike traits in the skull, teeth and pelvis. The boy's skull has
a volume of 420 cc, and both fossils are short, about 130 cm (4'3").

Australopithecus afarensis and africanus, and the other
species above, are known as gracile australopithecines, because their skulls
and teeth are not as large and strong as those of the following species, which
are known as the robust australopithecines. (Gracile
means "slender", and in paleoanthropology is used as an antonym to
"robust".) Despite this, they were still more robust than modern humans.

A. aethiopicus existed between 2.6 and 2.3 million years ago. This species
is known from one major specimen, the Black Skull discovered by Alan Walker, and a
few other minor specimens which may belong to the same species. It may be an
ancestor of robustus and boisei, but it has a baffling mixture of
primitive and advanced traits. The brain size is very small, at 410 cc, and parts
of the skull, particularly the hind portions, are very primitive, most resembling
afarensis. Other characteristics, like the massiveness of the face, jaws and
single tooth found, and the largest sagittal crest in any known hominid, are more
reminiscent of A. boisei (Leakey and Lewin 1992). (A sagittal crest is a
bony ridge on top of the skull to which chewing muscles attach.)

A. robustus had a body similar to that of africanus, but a larger and
more robust skull and teeth. It existed between 2 and 1.5 million years ago. The
massive face is flat or dished, with no forehead and large brow ridges. It has
relatively small front teeth, but massive grinding teeth in a large lower jaw. Most
specimens have sagittal crests. Its diet would have been mostly coarse, tough food
that needed a lot of chewing. The average brain size is about 530 cc. Bones
excavated with robustus skeletons indicate that they may have been used as
digging tools.

A. boisei existed between 2.1 and 1.1 million years ago. It was similar to
robustus, but the face and cheek teeth were even more massive, some molars
being up to 2 cm across. The brain size is very similar to robustus, about
530 cc. A few experts consider boisei and robustus to be variants of
the same species.

Australopithecus aethiopicus, robustus and boisei are known as
robust australopithecines, because their skulls in particular are more heavily
built. They have never been serious candidates for being direct human ancestors.
Many authorities now classify them in the genus Paranthropus.

H. habilis, "handy man", was so called because of evidence of tools found
with its remains. Habilis existed between 2.4 and 1.5 million years ago. It
is very similar to australopithecines in many ways. The face is still primitive,
but it projects less than in A. africanus. The back teeth are smaller, but
still considerably larger than in modern humans. The average brain size, at 650 cc,
is considerably larger than in australopithecines. Brain size varies between 500
and 800 cc, overlapping the australopithecines at the low end and H. erectus
at the high end. The brain shape is also more humanlike. The bulge of Broca's
area, essential for speech, is visible in one habilis brain cast, and
indicates it was possibly capable of rudimentary speech.
Habilis is thought to have been about 127 cm (5'0") tall, and about 45 kg
(100 lb) in weight, although females may have been smaller.

Habilis has been a controversial species. Originally, some
scientists did not accept its validity, believing that all habilis
specimens should be assigned to either the australopithecines or Homo
erectus. H. habilis is now fully accepted as a species, but it
is widely thought that the 'habilis' specimens have too wide a range of
variation for a single species, and that some of the specimens should be
placed in one or more other species. One suggested species which is
accepted by many scientists is Homo rudolfensis, which would contain
fossils such as ER 1470.

This species was named in 2002 to contain fossils found in Dmanisi, Georgia, which seem intermediate between H. habilis and H. erectus. The fossils are about 1.8 million years old, consisting of three partial skulls and three lower jaws. The brain sizes of the skulls vary from 600 to 780 cc. The height, as estimated from a foot bone, would have been about 1.5 m (4'11"). A partial skeleton was also discovered in 2001 but no details are available on it yet. (Vekua et al. 2002, Gabunia et al. 2002)

H. erectus existed between 1.8 million and 300,000 years ago. Like
habilis, the face has protruding jaws with large molars, no chin, thick
brow ridges, and a long low skull, with a brain size varying between 750
and 1225 cc. Early erectus specimens average about 900 cc, while
late ones have an average of about 1100 cc (Leakey 1994). The skeleton is
more robust than those of modern humans, implying greater strength. Body
proportions vary; the Turkana Boy is
tall and slender (though still extraordinarily strong), like modern humans
from the same area, while the few limb bones found of Peking Man indicate a shorter, sturdier
build. Study of the Turkana Boy skeleton indicates that erectus may
have been more efficient at walking than modern humans, whose skeletons
have had to adapt to allow for the birth of larger-brained infants (Willis
1989). Homo habilis and all the australopithecines are found only
in Africa, but erectus was wide-ranging, and has been found in
Africa, Asia, and Europe. There is evidence
that erectus probably used fire, and their
stone tools are more sophisticated than those
of habilis.

Some scientists classify some African erectus specimens as belonging to a separate species, Homo ergaster, which differs from the Asian H. erectus fossils in some details of the skull (e.g. the brow ridges differ in shape, and erectus would have a larger brain size). Under this scheme, H. ergaster would include fossils such as the Turkana boy and ER 3733.

Homo antecessor was named in 1977 from fossils found at the Spanish cave site of
Atapuerca, dated to at least 780,000 years ago, making them the oldest confirmed
European hominids. The mid-facial area of antecessor seems very modern, but
other parts of the skull such as the teeth, forehead and browridges are much more
primitive. Many scientists are doubtful about the validity of
antecessor, partly because its definition is based on a juvenile specimen, and
feel it may belong to another species. (Bermudez de Castro et al. 1997; Kunzig
1997, Carbonell et al. 1995)

Archaic forms of Homo sapiens first appear about 500,000 years ago. The term
covers a diverse group of skulls which have features of both Homo erectus and
modern humans. The brain size is larger than erectus and smaller than most
modern humans, averaging about 1200 cc, and the skull is more rounded than in
erectus. The skeleton and teeth are usually less robust than erectus, but
more robust than modern humans. Many still have large brow ridges and receding
foreheads and chins. There is no clear dividing line between late erectus
and archaic sapiens, and many fossils between 500,000 and 200,000 years ago
are difficult to classify as one or the other.

Neandertal (or Neanderthal) man existed between 230,000 and
30,000 years ago. The average brain size is slightly larger than that of
modern humans, about 1450 cc, but this is probably correlated with their
greater bulk. The brain case however is longer and lower than that of
modern humans, with a marked bulge at the back of the skull. Like
erectus, they had a protruding jaw and receding forehead. The chin
was usually weak. The midfacial area also protrudes, a feature that is not
found in erectus or sapiens and may be an adaptation to cold.
There are other minor anatomical differences from modern humans, the most
unusual being some peculiarities of the shoulder blade, and of the pubic
bone in the pelvis. Neandertals mostly lived in cold climates, and their
body proportions are similar to those of modern cold-adapted peoples: short
and solid, with short limbs. Men averaged about 168 cm (5'6") in height.
Their bones are thick and heavy, and show signs of powerful muscle
attachments. Neandertals would have been extraordinarily strong by modern
standards, and their skeletons show that they endured
brutally hard lives. A large number of tools and weapons have been
found, more advanced than those of Homo erectus. Neandertals were
formidable hunters, and are the first people known to have buried their
dead, with the oldest known burial site being about 100,000 years old.
They are found throughout Europe and the Middle East. Western European
Neandertals usually have a more robust form, and are sometimes called
"classic Neandertals". Neandertals found elsewhere tend to be less
excessively robust. (Trinkaus and Shipman 1992; Trinkaus and Howells 1979;
Gore 1996)

Homo floresiensis was discovered on the Indonesian island of Flores in 2003. Fossils have been discovered from a number of individuals. The most complete fossil is of an adult female about 1 meter tall with a brain size of 417cc. Other fossils indicate that this was a normal size for floresiensis. It is thought that floresiensis is a dwarf form of Homo erectus - it is not uncommon for dwarf forms of large mammals to evolve on islands. H. floresiensis was fully bipedal, used stone tools and fire, and hunted dwarf elephants also found on the island. (Brown et al. 2004, Morwood et al. 2004, Lahr and Foley 2004)

Modern forms of Homo sapiens first appear about 195,000 years ago.
Modern humans have an average brain size of about 1350 cc. The forehead
rises sharply, eyebrow ridges are very small or more usually absent, the
chin is prominent, and the skeleton is very gracile. About 40,000 years
ago, with the appearance of the Cro-Magnon culture, tool
kits started becoming markedly more
sophisticated, using a wider variety of raw materials such as bone and
antler, and containing new implements for making clothing, engraving and
sculpting. Fine artwork, in the form of decorated tools, beads, ivory
carvings of humans and animals, clay figurines, musical instruments, and
spectacular
cave paintings appeared over the next 20,000 years. (Leakey 1994)

Even within the last 100,000 years, the long-term trends towards smaller molars and
decreased robustness can be discerned. The face, jaw and teeth of Mesolithic humans
(about 10,000 years ago) are about 10% more robust than ours. Upper Paleolithic
humans (about 30,000 years ago) are about 20 to 30% more robust than the modern
condition in Europe and Asia. These are considered modern humans, although they are
sometimes termed "primitive". Interestingly, some modern humans (aboriginal
Australians) have tooth sizes more typical of archaic sapiens. The smallest
tooth sizes are found in those areas where food-processing techniques have been used
for the longest time. This is a probable example of natural selection which has
occurred within the last 10,000 years (Brace 1983).